The present invention relates to the field of radiofrequency reception, and more particularly to a selector fitted in a frequency-modulation radiofrequency receiver for selecting a channel.
In a known manner, a frequency-modulation radiofrequency signal uses a center frequency characteristic of a channel, and encodes its information by varying the frequency around said center frequency.
In many countries, the frequency-modulation frequency plan has center frequencies every 200 kHz. As illustrated in FIG. 1, showing the power P as a function of the frequency F, a channel C0 thus has a total modulation range EMT of between 100 kHz below the center frequency F0 and 100 kHz above the center frequency F0. In practice, in order to avoid risks of overlapping, and to facilitate the extraction, or the selection, of a channel from an entire received signal that may contain a multitude of channels, the modulation range is limited to a useful modulation range EMU, typically equal to 75% of the total modulation range EMT, i.e. a width of 150 kHz, centered around the center frequency F0, i.e. a useful modulation range EMU of between 75 kHz below the center frequency F0 and 75 kHz above the center frequency F0. Organizing and using the frequency spectrum in this way implies a comfortable margin MA between a channel C0 and an adjacent channel C1. A channel C0 is typically selected by means of a bandpass filter or selector filter 2, the frequency band BPF of which is centered on the center frequency F0 of the channel C0 and has a width corresponding to the width of the total modulation range EMT, i.e. 200 kHz in the previous example. This width BPF is preferably able to be limited to the width of the useful modulation range EMU, i.e. 150 kHz in the previous example.
The selector filter 2 receives, as input, an initial signal 8 (cf. FIG. 5), which may be a base-frequency signal, originating directly from the antenna, or a signal whose frequency has already been lowered, such as an intermediate-frequency signal. It produces, as output, a signal 9 that is homologous, but confined only to the channel C0, in the frequency band BPF. This output signal 9 is able to be processed by a demodulator.
However, in practice, for various reasons such as variations in the frequency plan from one country to another, or indeed local conditions of implementation, mean that a signal may impinge on the modulation range EMU, EMT of the channel C0. Such a signal then becomes an interference signal 3 in relation to the channel C0.
The result of an interference signal 3, here a signal e1, being present in the useful modulation range EMU of a channel C0 conveying a signal e0 is illustrated in FIGS. 3 and 4. FIG. 3 shows a filter 2, the frequency band BPF of which is tailored to the useful modulation range EMU of the channel C0. This frequency band BPF is centered on the center frequency F0 of the channel C0 and effectively encompasses the signal e0. However, on account of an excessively wide frequency band BPF, the filter 2 still at least partially retains the interference signal e1. As illustrated in FIG. 4, the portion of the interference signal e1 retained by the filter 2 is superimposed on the signal e0. The resultant signal, which is highly distorted, produces an audio output that is highly unpleasant to the ear.
It is therefore advantageous, for a selector 1, to reduce the width of the frequency band BPF of the filter 2 when an interference signal 3 is present in the useful modulation range EMU, in order to eject said interference signal 3 from the frequency band BPF retained by the filter 2. Thus, as illustrated in FIG. 2, a frequency band BPF filter 2 confined to the envelope shown in an unbroken line makes it possible to effectively eliminate the interference signal 3.
However, it is appreciated that reducing the frequency band BPF in this manner is detrimental, in that it eliminates a portion of the signal e0 from the channel C0, and is particularly undesirable in the absence of interference 3.
Therefore, according to prior art illustrated in FIG. 5, it is known to produce a selector 1, the frequency band BPF of the bandpass filter 2 of which is centered on the center frequency F0 of the channel C0 to be selected and has a width that is able to be adjusted automatically. This enables the selector 1 to adapt to changing environmental radiofrequency conditions. Such a selector 1 thus determines, continuously and in an adaptive manner, a frequency bandwidth BPF of the filter 2 depending on measurements that are indicative of the presence or of the absence of interference, and that is able to situate said interference outside of said frequency band.
One example of this prior art is a device 4, for example in the form of an integrated circuit 4, with the reference SAF7741HV, produced by NXP. The content of such a device 4 remains proprietary and is known only in ‘black box’ mode, by its inputs and outputs. As output, the device 4 produces a frequency band BPF for a selector filter 2, or, more precisely, the center frequency F0 remaining identical for a given channel C0, the device 4 produces a width of this frequency band BPF. This output is determined depending on the inputs. As input, the device 4 receives a signal B indicative of the noise level present in the channel C0, typically provided by a noise level sensor 7 working on the signal 9 at the output of the selector 1, a signal M indicative of the modulation level of the channel C0, typically provided by a modulation level sensor 6 working on the signal 9 at the output of the selector 1, and a signal C indicative of the received field level for the channel C0, typically provided by a field level sensor 5 working on the signal 9 at the output of the selector 1. The exact algorithm for transforming the inputs to determine the output (width of BPF) is not known. It is known, however, that the width at the output is an increasing function of the modulation level M, a decreasing function of the noise level B, and an increasing function of the field level C. The width of the frequency band BPF thus increases with the modulation level M, but is reduced, in contrast, when the noise B increases and/or when the field level C decreases very substantially.
It should be noted that, in the prior art, the modulation level M is used to determine the weighting defining the filter 2. In no case is the modulation level M used to determine the minimum value of the filter 2.
The device 4 also receives other parameters, which are not shown, as input. It may thus include temporal parameters, making it possible to configure the attack and release time constants Ta and Tr, when transients occur. Regulating these parameters makes it possible to achieve stability in the control determining the width of the frequency band BPF.
In order to limit the range of variation in the width of the frequency band BPF and to prevent excessively significant drift, the device 4 also receives, as input, a minimum value MinBP0 of the width of the frequency band BPF and a maximum value MaxBP0 of the width of the frequency band BPF. These two values MinBP0, MaxBP0 are used as boundaries, min and max, respectively, in order to saturate the frequency bandwidth BPF. In the prior art, these two values MinBP0, MaxBP0 are constants.
The maximum value MaxBP0 is typically taken to be equal to the useful modulation range EMU of the channel C0. This is a value of 150 kHz, looking again at the previous example.
The minimum value MinBP0 results from a compromise. The smaller this minimum value MinBP0, the more it is possible to continue to select a channel C0 despite a high level of interference B, but the more the channel C0 thus selected will have a tendency to produce an impaired audio output. If this value MinBP0 is greater, the channel C0 will produce a less impaired audio output on average, but the selection will be cut off at a lower level of interference B.
Therefore, a compromise is reached depending on the client; depending on the type of radiofrequency environment that it encounters and, above all, depending on its attitude toward tolerating a distorted audio output as opposed to the cutoffs in reception. The set minimum value MinBP0 is defined in the factory by the manufacturer, depending on the client and/or on the market, at a set adjustment value. In practice, with the numerical values from the previous example, the minimum value MinBP0 of the width of the frequency band BPF typically varies between 30 and 50 kHz.